Analytical devices based on diffusion boundary layer calibration and quantitative sorption

ABSTRACT

A method and device for determining the concentration of analytes in a sample. The device has a surface with an extraction coating thereon. The method is carried out while the sample is highly agitated to maintain a substantially constant boundary layer. The sample is brought into contact with the coating for a limited time so that all the analytes that pass through the boundary layer are adsorbed by the coating. The amount of analytes in the coating is determined and the concentration of those same analytes in the sample is then calculated from the diffusion coefficient of the analytes. The device is first calibrated using analytes of known concentration.

This application claims benefit of Provisional Application No.60/179,755 filed Feb. 2, 2000 and claims benefit of No. 60/196,587 filedApr. 13, 2000.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a method and device of determining theconcentration of analytes in sample using an extraction device wherebythe concentration of analytes of interest can be determined from thediffusion coefficient for said analytes. More particularly, thisinvention relates to a method and device for determining theconcentration of organic and inorganic compounds in liquid and gaseoussamples.

2. Description of the Prior Art

It is known to use solid phase microextraction (SPME) andpolydimethylsiloxane (PDMS) coated fibers to extract volatile organiccompounds (VOC's) in environmental samples. PDMS is the most widely usedcoating for extracting nonpolar volatile analytes as well as many polaranalytes. However, the sensitivity of mixed phase SPME coatings, such asPDMS/DVB and Carboxen/PDMS was reported to be much higher compared toPDMS coating for extracting VOC's (see Mani et al, Applications of SolidPhase Microextraction, RSC, Cornwall, U. K., 1999, Chapter 5). Mixedphase coatings have some complementary properties compared to PDMS andare more suitable for sampling highly volatile species (see Pawliszyn,Solid-Phase Microextraction: Theory and Practice, Wiley-VCH, Inc., NewYork, 1997, Chapter 4). Mixed phase SPME fibers have been used forsampling and quantifying target VOC's present in indoor air at the partper billion level and even at the part per trillium level.

Indoor air quality and its potential impact on human health is anincreased concern to the public and government environmental agencies.Many VOC's, such as formaldehyde, aromatic compounds, and halogenatedhydrocarbons have been found to be highly toxic to humans. Large-scaleair quality testing by conventional air sampling methods can often betime-consuming and expensive. Solid phase microextraction coupled withgas chromatography has been previously successfully applied to analyzevarious air samples. Chai et al., Analyst, 1993, 118,1501 reported thedetermination of the presence of volatile chlorinated hydrocarbons inair by SPME in 1993. Martos et al. developed a new method using a lineartemperature-programmed retention index method to calibrate SPME devicesfor fluctuations in sampling temperature, and for air analysis of totalVOC's with (PDMS) fibers (see Martos et al., Analytical Chemistry, 1997,69, 206 and 402). Grote et al used SPME for fast quantitative analysisof acetone, isoprene and ethanol in human breath with SPME fibers inAnalytical Chemistry, 1997, 69, 587. it is known that the syringe-likeSPME device is portable and can be easily used for field analysis. Whencoupled with a field-portable gas chromatograph, both SPME sampling andinstrumental analysis can be conducted at the test site without the needfor sample preservation (see Koziel et al, Analytical Chemistry, Acta,1999, 400(1-3), 153.

In mixed phase coatings, the majority of interaction on porous polymerparticles is determined by the adsorption process. With mixed phasecoatings, the molecules can be attracted to a solid surface via van derWaals, dipole—dipole, and other weak intermolecular forces (see Góreckiet al, Applications of Solid Phase Microextraction, RSC, Cornwall, U K,1999, Chapter 7). Hydrophobic interaction and electrostatic interactionalso occur when extracting analytes from water and ionizable analytesfrom aqueous phase, respectively. Compared to the diffusion coefficientin liquid coatings of PDMS or PA, the diffusion coefficients of VOC's indivinylbezene and Carboxen are so small that, within the frame of SPMEanalysis, essentially all the molecules remain on the surface of thecoating. Therefore, the fundamental difference between adsorption andabsorption is that in adsorption molecules bind directly to the surfaceof a solid phase while, in absorption, they dissolve into the bulk ofthe liquid phase.

The Langmuir adsorption isotherm is one of most important adsorptiontheories. The Langmuir model assumes there is only a limited number ofsurface sites that can be occupied by analyte molecules, all sites areequivalent, and there is no interaction between adsorbate molecules onadjacent sites. The Langmuir adsorption isotherm was used to describethe adsorption equilibrium on PDMS/DVB and Carbowax/DVB coatings. Alinear function is found to exist only if the affinity of an analytetoward the coating is low or its concentration in the sample is verylow. In a real sample matrix, for example, air, there are usually morethan two components. Since different components have differentaffinities towards the active sites, the presence of multi-componentsmust affect the adsorption of one other. Unlike the non-competitiveabsorption process in liquid coatings, adsorption process onto porouspolymer coatings in a multi-component system is a competitive processand therefore displacement effect is expected. Sampling conditions, thesample matrix composition and concentration can largely affect theamount of analytes extracted by mixed phase fibers. From a practicalpoint of view, this makes quantitative analysis using porous polymerSPME coatings more difficult.

The majority of adsorption models are based on the equilibrium theory.In SPME, however, the equilibrium time ranges from a few minutes to acouple of hours depending on the nature of the analytes and the samplingconditions (see Ai et al., Applications of Solid Phase Microextraction,RSC, Cornwall, U K, 1999, Chapter 2). For porous solid coatings, theequilibrium time for the same analyte is usually much longer than thatin liquid coatings. It may be impractical to wait for partitionequilibrium of all of analytes in the matrix if the equilibrium timesfor some analytes are too long.

In the direct SPME system, such as sampling in air or in water, theanalyte movement proceeds in two steps. The first step consists of themass transfer of analytes from the bulk sample matrix to the surface tothe SPME polymer coating followed by diffusion of the analytes withinthe coating. Fick's first law of diffusion (equation 1) can describe therate of mass diffusion in the sample matrix in the coating as follows:$\begin{matrix}{F = {- {D_{s}\left( \frac{\partial C_{s}}{\partial x} \right)}}} & \left( {{eq}.\quad 1} \right)\end{matrix}$

Where F is the flux of analyte in the direction x from the sample matrixbulk to the SPME fiber surface.

(i) D_(s) is the diffusion coefficient of the analyte in the samplematrix,

(ii) C_(s) is the analyte concentration in the sample bulk.

In a static gas system, mass movement results only from moleculardiffusion due to intermolecular collisions. In practice, both moleculardiffusion and bulk fluid movement must be considered. The extent offluid movement (agitation), reflects the access of analytes to thesurface and is frequently described as a theoretical parameter calledthe boundary layer (Cooper et al, Air Pollution Control: A DesignApproach, Waveland Press Inc., Prospect Heights, 1994, Chapter 13).

According to the boundary layer theory, a laminar sublayer or the samplematrix film is formed when a fluid passes a fixed object. The only waythat the analyte can pass from the air bulk phase to the surface of thecoating is via molecular diffusion across the boundary layer. In theliquid/solid interface, the thickness of the boundary layer isdetermined by the agitation conditions and the viscosity of the fluid(see Pawliszyn, Solid Phase Microextraction: Theory and Practice,Wiley/VCH, Inc., New York, 1997).

In a gas system, air wind velocity is a very important factor in masstransfer process. Because the value of wind velocity represents thedegree of bulk air movement, wind velocity will influence the overallmass transfer rate in the bulk of fluid. Based on mass transfertheories, the mass transfer rate of an analyte is proportional to themass diffusivity, and inversely proportional to the thickness of gasfilm at the interface.

Many factors such as temperature, pressure, molecular structure andmolecular weight can directly affect the molecular diffusioncoefficients of VOC's (see Lugg, G. A., Analytical Chemistry, 1968, 40(7), 1072). Since accurate experimental measurement of the diffusioncoefficient is difficult, relatively few values for organic compounds ingas systems are available from the literature. A number of methods havebeen proposed for estimation of diffusion coefficients of VOC's in airsystems. The method by Fuller, Schettler and Giddings (FSG method) wasreported to be most accurate for non-polar organic gases at low tomoderate temperature (see Lyman et al, Handbook of Chemical PropertyEstimation Method, ACS, McGraw-Hill, Inc., New York, 1982 Chapter 17).Minimal error is associated with the aliphatics and aromatics. FSG modeldescribes that the molecular diffusion coefficient of an analyte isdirectly proportional to temperature, and inversely proportioned to airpressure. The relative humidity of air is another factor that can affectVOC extraction on SPME fibers because water molecules participate in theadsorption process.

SUMMARY OF THE INVENTION

A method of determining a concentration of analytes of interest in asample using a solid phase microextraction device having a surfacecontaining an extraction coating comprises bringing the sample intocontact with the coating while highly agitating the sample undercontrolled conditions to maintain a substantially constant boundarylayer between the sample and the coating. The method further compriseslimiting a time of contact between the sample and the coating and sizingthe coating so that all analytes that pass through the boundary layerare adsorbed by the coating. The method further comprises terminatingthe contact, determining the amount of each analyte of interest in thecoating and calculating the concentration of each analyte of interest inthe sample by using the diffusion coefficient for that analyte.

A method of determining the concentration of analytes of interest in asample uses an extraction device having a membrane. The method comprisesbringing the sample into contact with the membrane for sufficient timeto allow microextraction to occur. The method further comprises choosinga membrane with a large surface area and limiting the time of contact sothat all analytes that contact the membrane are adsorbed by themembrane. The method further comprises separating the membrane from thesample, determining the amount of each analyte of interest in themembrane and calculating the concentration of each analyte of interestin the sample using the diffusion coefficient for that analyte.

A device for determining the concentration of analytes of interest in asample, the device comprises a solid phase microextraction device havinga surface containing an extraction coating characterized by a largesurface area to adsorb all analytes that contact said coating andagitation means to highly agitate the sample under controlled conditionsduring microextraction.

In a further embodiment, a device for determining the concentration ofanalytes of interest in a sample uses a membrane having a large surfacearea to adsorb all analytes that contact a surface of the membrane in atime allowed for extraction. The membrane is sized and shaped to fitinto an injection port of an analytical instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of an air sampling system;

FIG. 2 is a further embodiment of an air sampling system for sampleshaving varying humidities;

FIG. 3 is a graph showing the effect of wind velocity on adsorption;

FIG. 4 is a graph showing the extraction time profiles of toluene underdifferent wind velocities;

FIG. 5 is a graph of the slopes of toluene extraction time profiles andwind velocity;

FIG. 6 shows extraction time profiles for difference concentrations ofbenzene at high wind velocities;

FIG. 7 is a graph of the amount of benzene adsorbed and concentrationafter one minute of sampling time;

FIG. 8 is a graph of the amount of adsorption with temperature;

FIG. 9 is a graph showing the relationship between the amount adsorbedat an exposure time of 5 seconds with increasing temperature;

FIG. 10 is a graph showing the amount of adsorption with temperature ata 10 second sampling time with a different fiber than that used for FIG.9;

FIG. 11 is a graph showing the amount of benzene adsorbed with time atdifferent humidities;

FIG. 12 is a graph showing the amount of toluene adsorbed with time atdifferent humidities;

FIG. 13 is a graph showing the amount of p-Xylene adsorbed with time atdifferent humidities;

FIG. 14 shows an extraction device having an electric blower to provideconstant air agitation;

FIG. 15A is a schematic side view of an extraction device having a fiberwith an extraction coating thereon;

FIG. 15B is a schematic side view of a flow tube having an extractioncoating on an inner surface thereof,

FIG. 15C is a schematic side view of a vessel having an extractioncoating on an interior surface of the vessel;

FIG. 15D is a schematic side view of a vessel containing particulateswith an extraction coating thereon;

FIG. 15E is a schematic side view of a vessel having a stirrer with anextraction coating on the stirrer;

FIG. 15F is a schematic side view of a vessel having a stirring bar withan extraction coating on said bar;

FIG. 16 is a schematic perspective view of a boundary layer surroundinga silica rod having an extraction coating thereon and a graph ofconcentration profile;

FIG. 17 is a perspective view of an expanded membrane attached to ahandle;

FIG. 18 is a perspective view of the membrane of FIG. 17 rolled aroundthe handle;

FIG. 19 is a schematic side view of a holder having three fibers; and

FIG. 20 is a schematic side view of a device for determining theconcentration of analytes in a liquid.

DESCRIPTION OF A PREFERRED EMBODIMENT

Compared to the diffusion coefficient and liquid coatings of PDMS or PA,the diffusion coefficients of VOC's in divinylbenzene and Carboxen areso small that, with the frame of SPME analysis, essentially all of themolecules remain on the surface of the coating. Sampling conditions, thesample matrix composition and concentration can largely effect theamount of analytes extracted by mixed phase fibers. From a practicalpoint of view, this makes quantitative analysis using porous polymerSPME coatings more difficult.

It has been found that with a very short exposure time (for example, oneminute), when using SPME with PDMS/DVB coating fibers for fast samplingand analysis of VOC's in indoor air, there is a linear relationshipbetween adsorption and concentration. Within a one minute samplingperiod, airborne benzene, toluene, ethylbenzene and p-xylene (BTEX)extracted on a PDMS/DVB fiber increased linearly with the sampling time.The short exposure time before equilibrium produces an advantage due tothe fact that the adsorption rate is controlled by diffusioncoefficients of analytes rather than their distribution constants.Because the differences between the diffusion coefficients of VOC's aremuch smaller compared to the differences of the distribution constantsare much smaller than the differences between the distributionconstants, all target VOC's with similar molecular weights producesimilar extraction rates when using a short sampling time.

The mass transfer parameters through a boundary layer should includeboth molecular diffusion and bulk fluid movement. When using a porouspolymer SPME coating for air sampling, it can be reasonably assumed thatall available analyte molecules are mobilized within a very shortexposure period. In other words, when the concentration of analyte onthe coating surface is far from the saturation point, all of the targetmolecules are immediately adsorbed as soon as they contact the surfaceof the porous solid extraction coating. If the matrix composition andsampling conditions are kept constant, it has been found that the rateof mass diffusion of analyte will be proportional to its mass diffusioncoefficient in the sample bulk within this short time period. Also, ithas been found that there exists a quantitative relationship between theamount of analytes adsorbed and concentration depending on the diffusioncoefficient of analytes when using a very short exposure time thatoccurs well before equilibrium.

In the gas-solid interface, the thickness of the gas film is largelyaffected by the air movement or by the wind velocity and the nature ofair. In a gas system, air wind velocity is a very important factor inthe mass transfer process. Since the value of wind velocity representsthe degree of bulk air movement, wind velocity will influence overallmass transfer rate in the bulk of the fluid. Based on mass transfertheories, the mass transfer rate of an analyte is proportional to themass diffusivity and inversely proportional to the thickness of the gasfilm at the interface. Therefore, when considering air sampling withporous SPME fibers, air wind velocity is a very important factor relatedto the adsorption process, especially for pre-equilibrium extraction.

With the present invention, some of the critical factors, includingair/wind velocity, sampling temperature and air relative humidity havebeen investigated in relation to the adsorption process of VOC's ontoporous polymer SPME coatings under non-equilibrium conditions.

Extraction Model Development. The solid SPME fiber coating can bemodeled as a long cylinder with length L, and outside and insidediameters of b and a, respectively (FIG. 1). When the coating is exposedto moving air, an interface (or boundary layer) with thickness δdevelops between the bulk of air and the idealized surface of the fiber.The analytes are transported from the bulk air to the surface of thecoating via molecular diffusion across the boundary layer. In mostcases, the molecular diffusion of analytes across the interface is therate-limiting step in the whole adsorption process.

The analyte concentration in the bulk air (Cg) can be consideredconstant when a short sampling time is used, and there is a constantsupply of an analyte via convection. These assumptions are true for mostcases of SPME air sampling, where the volume of air is much greater thenthan the volume of the interface, and the extraction process does notaffect the bulk air concentration. In addition, the SPME solid coatingcan be treated as a perfect sink. The adsorption binding isinstantaneous and the analyte concentration on the coating surface (CO)is far from saturation and can be assumed to be negligible for shortsampling times and relatively low analyte concentrations in a typicalair. These concentrations range from parts-per-trillion (by volume) toparts-per-million (by volume) for most VOCs of interest and typicalindustrial hygiene, indoor and ambient air concentrations. The analyteconcentration profile can be assumed to be linear from Cg to CO. Inaddition, the initial analyte concentration on the coating surface (CO)can be assumed to be equal to zero when extraction begins. Diffusioninside the pores of a solid coating controls mass transfer from b to a.

The mass of extracted analyte with sampling time can be derived usingthe analogy of heat transfer in a cylinder with inside and outsidediameters of b and δ, respectively, with a constant axial supply ofheat. The steady-state solution to heat transfer can be translated intoa mass transfer solution by replacing temperatures with concentrations,heat with flux of mass and heat transfer coefficient with gas-phasemolecular diffusion coefficient. As a result, the mass of extractedanalyte can be estimated from the following equation: $\begin{matrix}{{n(t)} = {\frac{2\pi \quad D_{g}L}{\ln \left( \frac{b + \delta}{b} \right)}{\int_{0}^{t}{{C_{g}(t)}\quad {t}}}}} & (1)\end{matrix}$

where: n is the mass of extracted analyte over sampling time (t) in ng;Dg is the gas-phase molecular diffusion coefficient (cm2/s); b is theoutside radius of the fiber coating (cm); L is the length of the coatedrod (cm); δ is the thickness of the boundary layer surrounding the fibercoating (cm); and Cg is analyte concentration in the bulk air (ng/mL).It can be assumed that the analyte concentration is constant for veryshort sampling times and therefore Equation 1 can be further reduced to:$\begin{matrix}{{n(t)} = {\frac{2\pi \quad D_{g}L}{\ln \left( \frac{b + \delta}{b} \right)}C_{g}t}} & (2)\end{matrix}$

where t is the sampling time (s). The fiber length and the outsidediameter of the fiber coating are constant for each type of the fiber.The nominal length for the 65 μm PDMS/DVB and the 75 μm Carboxen™/PDMScoatings is L=1 cm, and the outside diameter 2b=0.0240 cm (±10%) and2b=0.0260 cm (±10%), respectively.

It can be seen from Equation 2 that the amount of extracted mass isproportional to the sampling time, Dg for each analyte, bulk airconcentration, and inversely proportional to δ. This in turn allows forquantitative air analysis. Equation 2 can be modified to estimate theanalyte concentration in the air in ng/mL for rapid sampling with solidSPME coatings: $\begin{matrix}{C_{g} = \frac{n\quad {\ln \left( \frac{b + \delta}{b} \right)}}{2\pi \quad D_{g}{Lt}}} & (3)\end{matrix}$

The amount of extracted analyte (n) can be estimated from the detectorresponse.

For a special case, where the thickness of the boundary layer is muchsmaller than the outside radius of the fiber (δ<<b), the generalsolution can be reduced to a flat plate problem. For such condition, In(1+δ/b)≈δ/b, 2πbL=A, and Equation 2 simplifies to: $\begin{matrix}{{n(t)} = {\frac{D_{g}A}{\delta}C_{g}l}} & (4)\end{matrix}$

where A is the surface area of the sorbent. Equation 4 is analogous tothe mass uptake model for the TWA sampling with retracted SPME fiber,where the distance between the needle opening and the fiber (Z) isreplaced by δ. 11, 12

Under equal conditions, the amount of extracted mass will be greater foran analyte with a greater gas-phase molecular diffusion coefficient(Dg). This is consistent with the fact that the analyte with a greaterDg will cross the interface and reach the surface of the fiber coatingfaster. Values of Dg for each analyte can be found in the literature orestimated from physicochemical properties. A number of methods have beenproposed for estimation of diffusion coefficients of VOCs in airsystems. The method by Fuller, Schettler and Giddings (FSG) was reportedto be the most accurate $\begin{matrix}{D_{g} = \frac{0.001 \times T^{1.75} \times \sqrt{\frac{1}{M_{air}} + \frac{1}{M_{voc}}}}{{p\left\lbrack {\left( {\sum V_{air}} \right)^{\frac{1}{3}} + \left( {\sum V_{voc}} \right)^{\frac{1}{3}}} \right\rbrack}^{2}}} & (5)\end{matrix}$

for non-polar organic gases at low to moderate temperatures:

where Dg is expressed in cm2/s; T is the absolute temperature (K); Mair,Mvoc are molecular weights for air and VOC of interest (g/mol); p is theabsolute pressure (atm); Vair, Vvoc are the molar volumes of air and theVOC of interest (cm3/mol). According to the FSG model, Dg is directlyproportional to temperature and inversely proportional to air pressure.Because the atmospheric pressure changes are relatively low, the airtemperature is a more important factor than pressure when consideringair sampling. Regardless, both atmospheric pressure and air temperatureare routinely monitored during conventional air sampling.

The thickness of the boundary layer (δ) is a function of samplingconditions. The most important factors affecting δ are SPME coatingradius, air velocity, air temperature and D_(g) for each analyte. Theeffective thickness of the boundary layer is determined by both rate ofconvection and diffusion. As the analyte approaches the sorbent surface,the overall flux is increasingly more dependent of diffusion thanconvection. The analyte flux in the bulk sample is assumed to becontrolled by convection, whereas the analyte flux inside the boundarylayer region is assumed to be controlled by diffusion. The effectivethickness of the boundary layer can be described as the location wherethis transition occurs, i.e., where the flux towards δ (controlled byconvection) is equal to the flux towards the surface of the SPME coating(controlled by diffusion). In the Nernst model, the matrix within theboundary layer is stationary. Experimental research indicated thatconvection was also present inside the boundary layer. However, itseffects decreased with the distance to the solid surface. The effectivethickness of the boundary layer can be estimated using Equation 6,adapted from the heat transfer theory for an SPME fiber in a cross flow:$\begin{matrix}{\delta = {9.52\quad \frac{b}{{Re}^{0.62}{Sc}^{0.38}}}} & (6)\end{matrix}$

where Re is the Reynolds number=2ub/v; u is the linear air velocity(cm/s); v is the kinematic viscosity for air (cm²/s); Sc is the Schmidtnumber=v/D_(g). The effective thickness of the boundary layer inEquation 6 is a surrogate (or average) estimate and does not take intoaccount changes of the thickness that may occur when the flow separatesand/or a wake is formed. Equation 6 indicates that the thickness of theboundary layer will decrease with an increase of the linear airvelocity. Similarly, when air temperature (T_(g)) increases, thekinematic viscosity also increases. Since the kinematic viscosity termis present in the numerator of Re and in the denominator of Sc, theoverall effect on δ is small.

The gas-phase molecular diffusion coefficient (D_(g)) for each analyteis also an important parameter controlling δ. As illustrated in Equation6, the effective thickness of the boundary layer will be reduced foranalytes with lower D_(g). This can be explained considering that,analytes with low molecular weight will reach the coating surface fasterthen the less volatile analytes under equal experimental conditions andtherefore the point at which the diffusion is a primary mode of analytetransport to the coating is located further away from the surface. Thereduction of the boundary layer and the increase of the mass transferrate for an analyte can be achieved in at least two ways, i.e., byincreasing the air velocity and by increasing the air temperature.However, the temperature increase will reduce the solid sorbentefficiency. As a result, the sorbent coating may not behave as a zerosink for all analytes.

Chemicals and Supplies. The volatile organic compounds under study,i.e., benzene, toluene and p-xylenes were purchased from Sigma-Aldrich(Mississauga, ON). All VOC standards had purities ≧98.0% and used forcalibrating GC/FID response factors. National Institute of Standards andTechnology (NIST) traceable certified permeation tubes of benzene,toluene and p-xylene were purchased from Kin-Tech (La Marque, Tex.), andused for the generation of a standard gas mixture. Ultrahigh purityhydrogen, nitrogen, air were purchased from Praxair (Waterloo, ON). SPMEfibers with 65 μm PDMS/DVB, 75 μm Carboxen/PDMS and SPME holders werepurchased from Supelco (Oakville, ON).

Standard Gas. A standard gas-generating device with a flow-throughsampling chamber, was constructed to provide a wide range of target VOCconcentrations at constant temperature. Ultrahigh purity air (zero gas)was supplied from a Whatman air generator (Haverhill, Mass.) andmaintained at 50 psi head pressure. Permeation tubes of benzene, tolueneand p-xylene were held inside a glass permeation tube adapter (Kin-Tech,La Marque, Tex.) and swept with a constant flow of dilution air. Theadapter was placed inside a cylindrical aluminum oven, which was heatedby two heating elements (100 W), and its temperature was controlled byK-type thermocouple (Omega™, Stamford, Conn.) and an electronic heatcontrol device (Science Shops, the University of Waterloo, ON). The airflow rate was controlled by two Sidetrack™ mass flow controllers (SierraInstruments, Monterey, Calif.) placed on both the primary and thedilution loops in the system. Wide ranges of concentration for targetVOC's were obtained by adjusting both the air flow rate and thepermeation tube incubating temperature.

Design for Air Wind Velocity Study. As shown in FIG. 1, an air samplingsystem 2 consists of a main cylindrical glass chamber 4 and fouradditional cylindrical glass chambers 6, 8, 10, 12. All of the glasschambers 4, 6, 8, 10, 12 have different diameters with the chamber 4being the largest. The chamber 6 has the smallest diameter with thechambers 8, 10, 12 each increasing in diameter in chronological order.The main chamber 4 contains an SPME device 14, which is described inmore detail subsequently. A 1.0 L glass sampling bulb 16 (Supelco,Oakville, ON), was constructed and installed downstream from thestandard gas generator. The gas flow rate varied from 1,000 standardcubic centimeters per minute (sccm) to 4,000 sccm for generating a widerange of air wind velocities. This new sampling system can provide botha dynamic airflow with different wind velocities and a static gasmixture. Experiments for estimation of the range of air velocities intypical indoor environments were conducted in a mechanically ventilatedbuilding using an OMEGA™ HHF51 Temperature and Air Velocity Meter(OMEGA, Stamford, Conn.). The indoor air velocities were found to varywith the distance between the measured location and the air vent, whichis shown in the Table 1. The average indoor air velocity varied from 0to 10 cm/s, and the average wind velocity at ventilating zones (nearvent) varied from 15 to 40 cm/s. The data listed in Table 1 wereconsistent with the values reported by Wasiolek et al. (1999) IndoorAir-International Journal Indoor Air Quality and Climate, 9 (2) 125, whofound that the average indoor wind velocities (at 19 locations in aworkroom) varying from 1.4 to 9.7 cm/s, and the average wind velocity atthe breathing-zone height varying from 9.9 to 35.5 cm/s by using anaccurate three-dimensional sonic anemometer. The glass chambers 4, 6, 8,10, 12 allowed for sampling under dynamic flow conditions, and thechamber 16 allowed for static sampling when stopcocks 18 and 20 wereclosed. A stopcock 22 opens and closes an exhaust line 24. The averagewind velocities were calculated by dividing the airflow rate by thecross-section area of gas sampling chambers. When the air flow rate wasset at 1,000 sccm, the air velocities ranged from 0.2 to 20.8 cm/s. Whenthe air flow rate was increased to 4,000 sccm, the air velocities rangedfrom 0.8 to 83.2 cm/s. Since the Reynolds numbers in all chambers wereless than 1,200, the air flow in the sampling chambers was in a laminarflow condition. A 65 μm PDMS/DVB fiber was used to sample the VOC gasmixture in each sampling port under different average air velocities. Ashort exposure time of 20 seconds was used to examine the effect of windvelocity on the VOC adsorption process onto PDMS/DVB fiber. Theextraction time profiles of airborne BTEX were also constructed undervarious wind velocities.

Design for Temperature Study. The gas flow rate was maintained at 1,000sccm, and the permeation tubes were incubated at 60° C. The mainsampling chamber 4 in FIG. 1 was used to provide a steady-state massflow of VOC's at different temperatures. The air temperature in thevicinity of the SPME fibers was maintained within ±0.3° C. at the rangeof room temperature. A 65 μm PDMS/DVB fiber and a 75 μm Carboxen/PDMSfiber were used to sample the VOC gas mixture in the chamber. Thetemperature of the air stream in the chamber varied from 22 to 40° C.The SPME fiber exposure times were 5 seconds and 10 seconds,respectively.

Design for Air Humidity Study. As shown in FIG. 2, to create a dynamicairflow under different humidities, an in-line impinger trap 26(Supelco, Oakville, ON), and a humidity meter 28 (Radio Shack, Waterloo,ON) were installed in the air sampling system. The components of FIG. 2that are identical to the components of FIG. 1 are described using thesame reference numerals as those used in FIG. 1 without furtherdescription. Relative humidities of 47% and 75% were obtained bymaintaining the water level in the impinger trap at 1.0 cm and 8.0 cmheight, respectively. A 65 μm PDMS/DVB fiber was used to sample VOC's inthe gas mixture under different humidities.

Gas Chromatography. A Varian 3400 GC (Varian Associates, Sunnyvale,Calif.), equipped with a FID and a carbon dioxide-cooled septumprogrammable injector, was used to analyze air samples extracted by SPMEfibers and liquid samples of standard compounds. An SPB-5 capillarycolumn (30 m×0.25 mm i.d., 1.0 μm film thickness) was installed in theGC, and UHP helium was used as the carrier gas with a flow rate of 2.0mL/min at 26 psi head pressure. The oven temperature program was 50° C.for t min, 15° C./min to 240° C. and held for 2 min. For SPME fiberdesorption, the injector temperature was isothermally set at 300° C. forCarboxen/PDMS fibers, and at 250° C. for PDMS/DVB fibers. For liquidinjection, the injector was programmed from 45° C. to 225° C. at a rampof 300° C./min. The quantification of target VOC's in standard gases wasbased on the response factors obtained from the FID signals by liquidinjection of VOC standards in the test range.

Effects of Air Velocity. FIG. 3 shows the effect of wind velocity on theadsorption of benzene, toluene, p-xylene and ethylbenzene on a 75micrometers Carboxen/PDMS coating for 5s sampling of airborne BETEX.Each data point represents a normalized mass, i.e., the ratio ofadsorbed mass and the analyte concentration in air, and is shown with ±one standard deviation for three samples. FIG. 3 clearly indicates thattwo distinct regimes of mass transfer are present: regime (1) where theextracted amount depends on the air velocity and regime (2) where theair velocity has a less significant effect on the amount of extractedmass (“semi-plateau” region).

The two zone phenomena can be explained by considering an interfacebetween air and the porous solid sorbent. The first region in FIG. 3describes diffusion of analytes through the static, well-developedboundary layer surrounding the SPME coating. In this region, theincrease in air velocity causes a reduction in the boundary layerthickness and more of each analyte can be extracted per unit of time.This finding is consistent with the theory summarized by Equation 2. Inthe second region, above some critical velocity, the thickness of theboundary layer is further reduced, but it is small enough that the masstransfer is controlled by the diffusion inside the pores of the SPMEcoating. Therefore the increase in air velocity has only a small effecton the amount of extracted analyte.

The critical velocity for which the effects of the boundary layerthickness are negligible is approximately 10 cm/s for the analytes inthis study. Although this range is lower than the average air velocitiesin ambient air, the critical velocity is close to the range of measuredair velocities in typical indoor air. Reported average indoor airvelocities at the breathing-zone height varied from 9.9 to 35.5 cm/s,with the average of 19 locations in a workroom varying from 1.4 to 9.7cm/s. Particular care must be taken to ensure the reproducibility ofextraction conditions with porous SPME fibers in field sampling. This isbecause a small change in air velocity in the vicinity of solid SPMEfiber can have a significant effect on the amount of adsorbed analyte,particularly in the first mass transfer region (FIG. 3).

Considering the fact that the amount of extracted mass for solid SPMEfibers can be enhanced when sampling is conducted at greater airvelocities, i.e., in the “semi-plateau” region (FIG. 3), an external fanor an attachment to an air sampling pump can be used to provide greaterrate of mass transfer. Such a device could be used by air samplingprofessionals wishing to equalize the extraction conditions and providereproducible effective thickness of the boundary layer for each sample.The use of a higher air velocity for sampling with solid SPME coatingsleads to enhanced sensitivity. Preliminary results indicate that the useof solid PDMS/DVB 65 μm fiber coating, 30 s sampling and average airvelocity of 1 m/s allows for detection of BTEX at 10 ppt (by volume)range.

The greatest amount of mass was adsorbed for benzene, followed bytoluene, p-xylene and ethylbenzene. This finding is consistent withtheory presented in Equation 2, i.e., the mass of adsorbed analyte usingrapid sampling is proportional to the D_(g) for each analyte, when allother sampling conditions are equal. The 75 μm Carboxen™/PDMS coatingwas acting as a zero sink for short sampling times. The ratio ofnormalized masses in FIG. 3 for benzene and toluene was close to theratio of their D_(g)'s estimated by the FSG method. Normalized massesfor ethylbenzene and p-xylene were smaller than expected. Thisdiscrepancy is likely associated with experimental errors.

FIG. 4 shows the extraction time profiles of toluene using a 65 μmPDMS/DVB fiber under different wind velocities ranging from 0.8 to 83.2cm/s. These curves illustrate the variation of toluene uptake within thewhole range of indoor air wind speed. The toluene mass loading onPDMS/DVB fiber linearly increases with the sampling time within a shortperiod of time (1 min). Furthermore, within this short sampling time,the toluene uptake rises with the increase of wind velocity. However,the equilibrium mass loading of toluene generally decreases with theincrease of wind speed. This is caused by the fact that other, morestrongly bound compounds extract faster as well resulting in a fasteroccurring displacement effect.

A further examination of the wind speed effect indicates that there aredifferent influences on toluene adsorption on PDMS/DVB fiber whensampling at different wind velocities. Generally, the slope of thetoluene extraction time profiles increases with the increase of windvelocity. If we plot the slopes of toluene extraction time profilesagainst the average wind velocities applied, we can obtain FIG. 5. Thisfigure demonstrates that the slope increases approximately linearly asthe wind speed increases from 0.8 to 8.7 cm/s. This means that thetoluene mass loading on PDMS/DVB fiber was significantly affected by thevariation within the average indoor wind velocity range (0-10 cm/s), andan approximately linear increase of mass loading can be expected withinthis wind range. Only a slight increase of toluene extraction was foundas the wind speed increased from 8.7 to 83.2 cm/s. This range of windvelocity is usually found at indoor air ventilating zones. Thisindicates that the mass loading of toluene is only slightly affected bythe variation of wind speed within indoor air ventilating zones (>10cm/s).

Air Sampling under Wind Velocities Equal to or above a Critical AirVelocity. FIGS. 4 and 5 suggest that air sampling with a PDMS/DVB fibershould be conducted above some critical air velocity, above which themass transfer and the analyte uptake are not affected by air velocityvariation. FIG. 6 shows extraction time profiles for benzene using a 65μm PDMS/DVB fiber to sample a standard VOC gas mixture at windvelocities equal to or above 10.2 cm/s. Three VOC concentrations wereobtained by setting different incubation temperatures (35, 40 and 60°C.) for VOC permeation tubes and a constant air flow at 2,000 sccm.Another VOC concentration was generated by maintaining the incubationtemperature at 60° C. and increasing the air flow rate to 4,000 sccm.These curves illustrate that the benzene uptake increased with thesampling time before reaching its equilibrium level. The higher theconcentration, the less time was required for the PDMS/DVB fiber toreach the equilibrium. However, only within a very short sampling time(1 min), was benzene mass loading approximately linear with samplingtime. FIG. 7 shows that benzene uptake or response increased linearlywith the concentration when 1 min sampling time was used under an airvelocity equal to or above 10.2 cm/s. For other target VOC's, i.e.,toluene and p-xylene, similar results were also observed (data notshown).

Temperature Effect on VOC Adsorption on Porous SPME Fibers. FIG. 8 showsthat amounts of toluene and p-xylene adsorbed on the PDMS/DVB fiber fora 5s exposure increase linearly as the temperature increases from 22 to26° C., while benzene uptake remains almost constant in this temperaturerange. As the temperature increases continuously, the amounts of tolueneand p-xylene adsorbed increase slightly, while the amount of benzeneadsorbed decreases. This indicates the displacement of benzene moleculesby p-xylene or toluene molecules, which have higher affinities to thePDMS/DVB coating than benzene. The results indicate that increasingtemperature within a certain range will enhance the adsorption of VOC'son PDMS/DVB coating, especially for analytes with higher affinity to thecoating. Similarly to FIG. 8, FIG. 9 shows that the mass of toluene andp-xylene adsorbed onto the PDMS/DVB fiber for a 10s exposure increasewith the increase of temperature from 22 to 25° C. Benzene adsorbedremains almost constant from 22 to 25° C., but decreases as thetemperature increases further from 25 to 40° C. In fact, this situationshould be expected because the active surface sites become saturated asthe adsorption on the coating proceeds, and some benzene molecules arepossibly displaced by toluene or p-xylene molecules.

In FIG. 10, amounts of all three analytes adsorbed on the Carboxen/PDMSfiber increase linearly as the temperature increases from 22 to 25° C.when using 10s sampling time. As the temperature increases continuously,the mass of analytes adsorbed only increases slightly. Generally, asimilar adsorption behavior between PDMS/DVB and Carboxen/PDMS wasobserved. However, the Carboxen/PDMS fiber has a higher adsorptioncapacity than PDMS/DVB fiber for extraction of benzene, the smallestmolecule among the analytes. Unlike the DVB particle consisting ofmainly mesopores and a smaller fraction of macropores and micropores,the Carboxen polymer particle has an even distribution of micro, meso,and macro pores. Therefore, Carboxen particles are better for samplingsmaller molecules (C₂-C₁₂) compared to PDMS/DVB fiber. Unfortunately,the weakness of Carboxen/PDMS is the difficulty for analyte desorption.Peak tailing is often observed even with the GC injector temperature of300° C.

As one of the most important experimental parameters in SPME sampling,the extraction temperature has been discussed in several previous papersrelated to SPME air sampling. When a pure-phase liquid SPME fiber isused, an increase in extraction temperature usually causes an increasein extraction rate, but simultaneously a decrease in the distributionconstant. Since the extraction by the SPME coating is an exothermicprocess, a decrease in mass loading at equilibrium is usually expectedas the extraction temperature increases. In contrast to liquid fiber,however, an opposite trend of temperature effect was found in this studywhen mixed-phase porous SPME fibers were used. Within a very shortsampling time (far from equilibrium), VOC analytes on a porous SPMEfibre can linearly increase as the extraction temperature increases in anarrow range. Since VOC adsorption on a porous SPME fiber is controlledby the diffusion process or diffusion coefficient rather than theextraction equilibrium or distribution constant, an increase indiffusion coefficients with an elevated temperature should increase VOCuptake on the solid SPME coating.

Effect of Humidity on VOC Adsorption onto PDMS/DVB Fiber. FIGS. 11-13are the extraction time profiles of benzene, toluene and p-xylene usinga 65 μm PDMS/DVB fiber to extract a standard VOC gas mixture atdifferent humidities. These figures indicate that a humidity level of75% resulted in a significant decrease in the VOC uptake on the PDMS/DVBcoating at equilibrium, especially for smaller molecules, e.g., benzene.Due to the high affinity to the PDMS/DVB porous polymer coating, watermolecules compete with other VOC molecules and occupy a portion ofactive surface sites on the coating surface. Therefore, fewer activesurface sites are available to VOC molecules, especially to smallermolecules with lower affinity to the coating. However, within a veryshort sampling time, i.e., 1 minute, no significant difference wasobserved among the conditions with different humidities. This indicatesthat the active surface sites are not saturated within a very shortextraction time, and still available to VOC molecules. Thus, a shortsampling time (far before equilibrium) minimizes the effect of humidityon adsorption of VOC's on the PDMS/DVB coating. Table 4 shows the effectof relative humidity for decreasing VOC uptake onto the PDMS/DVBcoating. The humidity effect can be neglected if using PDMS/DVB fiberfor a very short time air sampling in a low humidity (<50%). However,the result suggests that a mass loading decrease of VOC on PDMS/DVB canbe expected if the relative humidity is above 50%.

Air wind velocity and temperature are important parameters related tothe diffusion process on porous polymer SPME fibers, particularly atnon-equilibrium conditions. Wind speed or bulk air movementsignificantly affects the VOC mass transfer process from the bulk air tothe fiber in a certain range. This indicates that the thickness of thegas-phase boundary layer between the fiber and air is diminished as thewind speed increases, and the mass transfer rate was accelerated between0 and 5 cm/s. This wind speed range is typical to average air velocitiesin indoor air. Therefore, air sampling with porous polymer coated SPMEfibers should be conducted above some critical air velocity, above whichthe mass transfer and the analyte uptake are not affected, and theextraction can be reproduced.

When using a liquid SPME fiber, a decrease in mass loading atequilibrium is usually expected as the extraction temperature increasesbecause the extraction is an exothermic process. An opposite trend oftemperature effect was found in this study when mixed-phase porous SPMEfibers were used with a short sampling time. Within a very shortsampling time far from equilibrium, analytes extracted on a porouspolymer coated SPME fiber increased linearly as the extractiontemperature increased in a narrow range from 22 to 25° C. In this case,the adsorption process is controlled by diffusion coefficients insteadof distribution constants of analytes. The effects of wind andtemperature on adsorption by porous polymer coated SPME fibers undernon-equilibrium conditions have not been addressed by previousresearchers. The analytical data indicate that there is a directrelationship between the rate of mass transfer and the analyte diffusioncoefficients. Therefore appropriate diffusion coefficients obtainedeither from the literature, calculated or experimentally determined canbe used to calibrate the relationship between amount of analyteextracted versus concentration for given extraction time. During theexperiment constant agitation condition are necessary, or if differentagitation conditions are used than the appropriate adjustmentcoefficients needs to be calculated. FIG. 14 shows the example of asimple device 34 based on an electric blower 36 having a fan (not shown)that sucks air into an inlet 38 on a cylindrical head 40 and exhauststhe air through an outlet 42. The flower 36 is able to provide constantair agitation. An SPME device 14 has a holder 44 mounted in an SPMEinsert 46. An O ring 48 is located between the holder 44 and insert 46.A sleeve 49 extends through the head 40 to position a fiber 50 withinthe head 40. The blower 36 has a handle 52. The reported data can beextended to liquid sample analysis. In this case, diffusion of analytesin a liquid matrix can be used to calibrate the response. Analogousdevices to one showed on FIG. 14 for air analysis can be designed toprovide constant agitation of the sample matrix. For solid samples,indirect headspace or liquid extraction can be used.

For air analysis, high humidity was found to decrease VOC uptake on thePDMS/DVB coating. However, the humidity effect can be minimized by usinga very short exposure time, in which case the active surface sites arenot saturated. The humidity effect can be neglected when using PDMS/DVBfiber for a very short time air-sampling in a low humidity, but a massloading decrease of VOC's on PDMS/DVB can be expected if air samples aretaken from a high humidity environment when the calibration might benecessary. It is expected that both temperature and humidity will bemonitored during the field measurement and appropriate correctioncoefficients will be calculated, if necessary to adjust the response.

The proposed diffusion based extraction and calibration approach isexpected to be the fastest possible sampling/sample preparation approachin the field and in the laboratory. Various different arrangements ofthe extraction phase can be used to practically implement thistechnology (see FIGS. 15A to 15E).

In FIG. 15A, a vessel 60 containing a sample 62 has an extraction phasecoating 63 on a tubular member 64. In FIG. 15B a tube 66 has anextraction phase coating 63 on an inner surface thereof. Sample 62contacts the extraction phase coating as it flows through the tube 66.

In FIG. 15C, the vessel 60 has an extraction phase coating 63 lining aninner surface thereof. The sample 62 contacts the coating 63 when it iscontained in the vessel.

In FIG. 15D, the vessel 60 contains the sample 62. Particles 72 arelocated within the sample and each particle is surrounded by extractionphase coating 63. In FIG. 15F, the vessel 60 has a sample 62 with astirrer 74 extending into the sample. The stirrer has paddles 76 withextraction phase coating 63 on the paddles. In FIG. 15F, there is showna vessel 60 containing a sample 62 with a stirring bar 78 located withinthe sample. The stirring bar 78 contains extraction phase coating 63.

In FIG. 16, there is shown a schematic perspective view of a silica rod60 surrounded by a solid extraction phase coating 82 that is porous andcontains pores 86. A cylindrically shaped boundary layer 84 surroundsthe cylindrically shaped coating 82 and analytes, designated by D_(g)pass through the boundary layer and are adsorbed by the coating 86. Agraph at the bottom of FIG. 16 shows that the boundary layer has athickness δ, the rod 80 has a radius a and a radius b equals thedistance from a center of the rod 80 to the exterior surface of thecoating 86. The concentration at C_(o) is the concentration at theinterface between the boundary layer and the coating and theconcentration at C_(g) is the concentration of the gas.

In FIG. 17, a membrane 90 is supported on a handle 92 and the membraneis in an unfolded position. In FIG. 18, the membrane 90 is rolled aroundthe handle 92. FIG. 17 shows the collection mode where the handle andmembrane are brought into contact with a sample (not shown). After rapidextraction has occurred, the membrane is moved out of contact with thesample and the membrane 90 is rolled around the handle 92 to make itmore compact. The membrane and handle can then be inserted into acylindrical sheath (not shown), which is airtight. The membrane can thenbe transferred to an analytical instrument, which can be located in thefield where the sample has been taken or to an instrument located awayfrom the test site. The sheath prevents the membrane from becomingcontaminated during transport.

In FIG. 19, there is shown a further embodiment of the invention where abrush 100 has bristles 102 extending therefrom and each bristle has anextraction phase coating 104 thereon. In FIG. 20, a device 107 has aSPME syringe 108 supporting a fibre 109 with a coating 110. A motor 112powers a stirrer 114 in a bracket 116.

I claim:
 1. A device for determining the concentration of analytes ofinterest in a sample, said device comprising a membrane having a largesurface area to adsorb all analytes that contact a surface of saidmembrane in a time allowed for extraction, said membrane having ahandle, said membrane being folded by rolling the membrane around thehandle, there being a sheath into which the rolled up membrane can beinserted said membrane being sized and shaped to fit into an injectionport of an analytical instrument.
 2. A device as claimed in claim 1wherein the membrane has a rectangular shape with a large surface arearelative to the analytes of interest that can be potentially adsorbedduring the time of exposure.
 3. A device as claimed in claim 1 whereinthe sheath can be sealed.
 4. A device as claimed in claim 3 wherein thedevice is portable.
 5. A device fin determining the concentration ofanalytes of interest in a sample, said device comprising a membranehaving a large surface area to adsorb all analytes that contact asurface of said membrane in a time allowed for extraction, said membranebeing a plurality of fibers connected to a holder, said fibers beingextendable and retractable into and out of said holder respectively,said fibers being sized and shaped to fit into an injection port of ananalytical instrument.